1. Field of the Invention
This invention relates to a tool for providing a visual indicia of the state of contacts of an interrupter assembly of a circuit breaker where the contacts are not readily visible, and more particularly to a contact erosion gauge for a vacuum circuit breaker assembly that houses the interrupter contacts in a hermetically sealed housing.
2. Background Information
Circuit breakers provide protection for electrical systems from electrical fault conditions such as current overloads, short circuits, and low level voltage conditions. Typically, circuit breakers include a spring-powered operating mechanism which opens electrical contacts to interrupt the current through the conductors in an electrical system in response to abnormal conditions. In particular, vacuum circuit interrupters include separable main contacts disposed within an insulated and hermetically sealed housing. Generally, one of the contacts is fixed relative to both the housing and to an external electrical conductor which is interconnected with the circuit to be protected by the circuit interrupter. The other contact is moveable. In the case of a vacuum circuit interrupter, the moveable contact assembly usually comprises a stem of circular cross-section having the contact at one end enclosed within the vacuum chamber, and a driving mechanism at the other end which is external to the vacuum chamber. In one type of vacuum interrupter, an operating rod assembly is provided which carries a rotatable contact bell crank which is coupled to the operating rod and rotates about a pivot pin upon motion of the operating rod. This plate is connected to the stem of the moveable contacts. The stem is typically affixed to a bellows seal which maintains the vacuum environment within the chamber while enabling movement of the stem and thus the moveable contact. Motion of the plate causes motion of the moveable contact into or out of engagement with the fixed contact.
The operating rod assembly is operatively connected to a latchable operating mechanism which is responsive to current. When an abnormal condition is reached, the latchable operating mechanism becomes unlatched which causes the operating rod to move to the open position. The motion of the operating rod, in turn, causes the contact bell crank to rotate and, as discussed above, this controls motion of the moveable contact and accommodates for wear on the contacts.
Compression springs are provided in the operating rod assembly in order to be able to separate the moveable contact from the fixed contact. In addition, when appropriate circumstances requiring interruption of the circuit do arise, an adequate force is needed to open the contacts with sufficient speed. If the contacts do not open quickly, there is a risk of failure to interrupt the current.
In order to achieve the adequate interrupt speed and force, the springs are mounted on the operating rod assembly. The springs are typically mounted towards one end of the operating rod on what is referred to as the contact wipe portion of the operating rod assembly. Contact wipe refers to the force required to hold the vacuum interrupter contacts closed and the energy to force the contacts open with sufficient speed for safe and clean interruption as discussed above. As noted above, the contact springs which comprise part of the contact wipe assembly must provide the force to hold the contacts closed and the energy to drive them open with appropriate speed. In a typical case, the spring is held on the operating rod between a disk-spacer member which is carried along by the operating rod and a shoulder portion of a set of contact wipe plates which are mounted at one end of the operating rod and spaced apart from the spacer member. When the contacts are closed, the operating rod travels toward its closed position. The contact wipe plates are slidably mounted on the operating rod at the same point at which the rotatable contact bell crank is mounted. When the contacts seat, motion of the contact wipe plates stop. However, motion of the operating rod continues until it travels to its full extended position. At this point, the spring is fully compressed between the spacer member and the shoulder section of the contact wipe plates.
U.S. Pat. No. 5,095,293 appears to describe the first known method of visually checking (without measurements) the spring compression (or tension) to determine whether it is adequate. Adequate spring compression is, in one respect, an indication of the contacts being in good condition. This is because contacts which are worn would require a greater degree of travel by the contact wipe plates which would mean that the compression of the springs between the shoulder portion of the contact wipe plates and the disk-shaped spacer would not be as great. U.S. Pat. No. 5,095,293 describes a convenient and easy to use inspection apparatus for use with the contact wipe springs from which the compression of the springs can be visually inspected. This serves as an indication of the correct contact force and indirectly provides an indication of the integrity of the contacts without requiring disassembly of the circuit breaker or any portion of it. The inspection apparatus employs an operating rod extension tongue received between the contact wipe plates. A visual indicia of the degree of motion between the contact wipe plates and the extension tongue of the operating rod provides a reading of adequacy of spring compression which, in turn, indirectly relates to the integrity of the contacts of the breaker.
While this indicia of contact wear functions well in the prior art vacuum interrupter design, more recent larger, higher voltage, more compact vacuum interrupter designs have found that extensions of the operating rods and/or stems are not readily visible outside the housing designs of these new breakers. Molded insulation casings housing these larger designs have, in general, made it more difficult to observe the moveable operating components within the high voltage portion of the breaker.
Accordingly, there is a need for a tool to measure contact wear that is more readily visible to maintenance personnel and will not interfere with the other operating components associated with the circuit interrupter nor inhibit replacement of the vacuum interrupter when it has expended its useful life.
There is a further need for such a tool to be inexpensive and easy to use.
There is a further need for such a tool to be compatible with existing circuit breakers.